Dual Type means 2 types. For and attack to be dual type it would have to be two types. Its not a misinterpretation. There are already moves that change element upon circumstances in the games. Nature Power, Nature's Gift, Secret Power, and Hidden Power. These are all classified as Normal attacks that can deal different typed Damage. The Damage type is what indicated STAB and Weakness/resistance so a Normal Pokemon using the attacks never get STAB or the attacks never get resisted or blocked by Normal Type's Resistance and Immunity checks despite them being Normal typed.

Yeah, I just posted what PokéBeach said about this, and I'll agree with you on this one.
I don't think we've misinterpreted it, nor do I think that dual-typed moves will exist, so I think he got this one wrong.